The present invention relates to the configuration of a passive optical network system in which multiple optical network units share an optical transmission line and a method for determining a fault occurred in the system.
Recently, FTTH (Fiber To The Home), which is an optical fiber service on access lines that serve subscribers, is introduced, and the communication services provided via this optical access network become more and more diversified as the transmission capacity grows.
An optical communication network called a Passive Optical Network (hereinafter referred to as a PON) has been widely used as an optical access network. This is because the PON system allows multiple users to share an optical fiber, the optical line terminator installed in a station receives communications from multiple users in the multiplexed mode, and the communication with each user's optical network unit can be managed integrally. Those features reduce the facility cost and the maintenance management cost of the optical access network.
Standardization work has been carried out as more and more PON systems are introduced, and GPON (Gigabit capable PON), which is standardized by International Telecommunication Union Telecommunication Standardization Sector (ITU-T), is widely used today as a standard PON (see Non-Patent Documents 1-3). This GPON, which uses 125-μsecond-period frames for data transmission control, can carry out variable-bandwidth, best-effort type communications such as constant-rate, constant-period information delivery via an E1 line or T1 line that has been used for conventional leased-line services or Ethernet (registered trademark: applicable also to the rest of this specification) that has been used for data communications. In addition to GPON, GEPON that supports best-effort type communications specifically designed for the Ethernet protocol is introduced and, to meet a future bandwidth demand, standardization work is underway for a PON that handles high-speed signals with the rate of 10 Gbits/second.
In a PON system, multiple Optical Network Units (hereinafter referred to as ONUs) installed in user sites and an Optical Line Terminator (hereinafter referred to as an OLT) installed in a communication carrier station are connected via optical fibers. A splitter is provided on the optical fiber that connects the ONUs and the OLT. Each ONU is connected to the splitter via a branch optical fiber, and the splitter is connected to the OLT via a trunk optical fiber. In the typical configuration of a PON system, the wavelength of an optical signal used for downlink communication and the wavelength of an optical signal used for uplink communication are different to avoid collision between the uplink communication and the downlink communication on the same optical fiber. The downlink signal from the OLT to an ONU is transferred in the PON section (the part including an optical fiber between the OLT and an ONU is sometimes called PON in the description below) as a form of a frame composed of a header and a payload, and the ONU references the header information to capture the necessary information. The uplink signals from all ONUs to the OLT are optical signals of the same wavelength, with a transmission time slot allocated to each ONU for time-division transmission to the OLT. The uplink signals sent from the ONUs via the branch optical fibers are once time-division multiplexed by the splitter onto the trunk optical fiber and are sent to the OLT.
Today, the PON system service is provided by distributing ONUs from a communication carrier to subscribers. In future, it is expected that a user will purchase an ONU and connect it to an optical connection outlet or an optical connection terminal device installed in the user's home. For example, a device in which a set-top box for receiving a TV broadcast via an optical network and an ONU are integrated will be sold at an ordinary electric store and a user who wants to subscribe to the PON system will purchase this device and connect the device to the carrier-provided OLT for using the PON system. In a case in which the user purchases an ONU as described above, a part of the optical section is released to the user, meaning that, depending upon the quality of an ONU provided by a third party other than a carrier and/or upon the user's usage method, there is a possibility that it will be difficult for the communication carrier to manage the ONUs or the whole PON system.
Optical line systems are standardized by documents, such as ITU-T Recommendation G984.1, ITU-T Recommendation G984.2, and ITU-T Recommendation G984.3.